1. Field of the Invention
The present invention relates to printing of patterns with extremely high precision on photosensitive surfaces, such as photomasks for semiconductor devices and displays. It also relates to direct writing of semiconductor device patterns, display panels, integrated optical devices and electronic interconnect structures. Furthermore, it can have applications to other types of precision printing such as security printing. The term printing should be understood in a broad sense, meaning exposure of photoresist and photographic emulsion, but also the action of light on other light sensitive media such as dry-process paper, by ablation or chemical processes activated by light or heat. Light is not limited to mean visible light, but a wide range of wavelengths from infrared (IR) to extreme UV. Of special importance is the ultraviolet range from 370 nm (UV) through deep ultraviolet (DUV), vacuum ultraviolet (WV) and extreme ultraviolet (EUV) down to a few manometers wavelength. EUV is in this application defined as the range from 100 nm and down as far as the radiation is possible to treat as light. A typical wavelength for EUV is 13 nm. IR is defined as 780 nm up to about 20 xcexcm.
In a different sense the present invention relates to the art and science of spatial light modulators and projection displays and printers using such modulators. In particular it improves the gray-scale properties, the image stability through focus and image uniformity and the data processing for such modulators by application of analog modulation technique. The most important use of the analog modulation is to generate an image in a high-contrast material such as photoresist with an address grid, i.e., the increment by which the position of an edge in the pattern is specified, that is much finer than the grid created by the pixels of the spatial light modulator.
2. Description of Background Art
Precision pattern generators using projection of micromirror spatial light modulators (SLMs) of the micromirror type are known in the art (Nelson, U.S. Pat. No. 5,148,157, 1988; Kxc3xcck, EP 0 610 183, 1990). The use of an SLM in a pattern generator has a number of advantages compared to the more wide-spread method of using scanning laser spots. Since an SLM is a massively parallel device, and the number of pixels that can be written per second is extremely high. The SLM optical system is also simpler in the sense that the illumination of the SLM is non-critical, while in a laser scanner the entire beam path has to be built with high precision. Compared to some types of scanners, in particular electrooptic and acoustooptic ones, the micromirror SLM can be used at shorter wavelengths since it is a purely reflective device.
In both references cited above the spatial light modulator uses only on-off modulation at each pixel. The input data is converted to a pixel map with one bit depth, i.e., with the values 0 and 1 in each pixel. The conversion can be done effectively using graphic processors or custom logic with area fill instructions.
In a previous application by the present inventor Sandstrxc3x6m (Sandstrxc3x6m et al., EP 0 467 076, 1990), the ability to use an intermediate exposure value at the boundary of a pattern element to fine-adjust the position of the element""s edge in the image created by a laser scanner was described.
Creation of a gray-scale image is also known in the art, preferably for projection display of video images and for printing. The is done with an SLM by variation of the time a pixel is turned on or by printing the same pixel several times with the pixel turned on a varying number of times. The present invention devices a system for direct gray-scale generation with a spatial light modulator, with a special view to the generation of ultra-precision patterns. Important aspects in the preferred embodiments, are uniformity of the image from pixel to pixel and independence of exact placement of a feature relative to the pixels of the SLM and stability when focus is changed, either with intention or inadvertently.
Specifically there is a problem with the prior art pattern generation to provide adequate image resolution and fidelity.
It is therefore an object of the present invention to provide an method to provide an increased resolution and fidelity.
This object is achieved with the method according to the appended claims.
The method according to the present invention comprises the steps of
providing a source for emitting electromagnetic radiation in the wavelength range from EW to IR,
illuminating by said radiation a spatial light modulator (SLM) having multitude of modulating elements (pixels),
projecting an image of the modulator on the workpiece,
moving said workpiece and/or projection system relative to each other,
further reading from an information storage device a digital description of the pattern to be written,
extracting from the pattern description a sequence of partial patterns,
converting said partial patterns to modulator signals, and feeding said signals to the modulator,
further coordinating the movement of the workpiece, the feeding of the signals to the modulator and the intensity of the radiation, so that said pattern is stitched together from the partial images created by the
sequence of partial patterns,
where an area of the pattern is exposed at least twice with a change in at least one, and preferably at least two, of the following parameters between the exposures:
data driven to the SLM
focus
angular distribution of the illumination at the SLM
pupil filtering
polarization.
By using several different exposures, the exposures could be adapted for certain features and aspects in the pattern, and hereby an improved resulting pattern is obtained. Thus, the method according to the present invention concentrates in a stepwise manner on one difficulty in the pattern at a time.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.